The Long Term Evolution (LTE) standard for wireless communication provides increasing capacity and speed compared to more conventional communication systems. The standard is specified, e.g., in the Release 10 document series of 3GPP.
LTE uses a frequency-division scheme for the uplink from user equipment (UE) to the base station. SC-FDMA is a frequency-division multiple access scheme. A multiple access scheme deals with the assignment of multiple users to a shared communication resource. SC-FDMA is related to OFDMA.
LTE defines three channels from the UE to the base-station: uplink control channel (physical uplink control channel), uplink traffic channel (Physical Uplink Shared Channel, PUSCH, based on SC-FDMA) and sounding reference signals (SRS). The frequencies used for the three uplink channels (PUSCH, PUCCH and SRS) have a common spacing (15 KHz). Moreover, the PUSCH, PUCCH and SRS channel also use the same time domain symbols grid. This means that the time domain to frequency domain conversion may be shared among them, i.e., the FFT and IFFT blocks.
LTE also defines a Random Access Channel (RACH). The RACH is used for initial access or when the UE losses its uplink synchronization. Information on RACH is encoded in a different manner. In LTE Zadoff-Chu sequences are used as a way to convey reference patterns. The RACH is used to get the attention of a base station in order to initially synchronize the device's transmission with the base station. With the RACH, in most cases, there is no certainty that only a single device makes a connection attempt at one time, so collisions can result.
The frequencies of the RACH are not orthogonal to those of the other three uplink channels, i.e., a different subcarrier spacing is used, 1.25 KHz, as opposed to 15 KHz for PUSCH, PUCCH and SRS. This means that the RACH requires its own FFT transform operation.